Enhancement of Superconductivity and Evidence of Structural Instability in Intercalated Graphite CaC 6 under High Pressure A. Gauzzi, 1,4, * S. Takashima, 1 N. Takeshita, 2 C. Terakura, 2 H. Takagi, 1,2 N. Emery, 3 C. He ´rold, 3 P. Lagrange, 3 and G. Loupias 4 1 Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8581, Japan 2 Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan 3 Laboratoire de Chimie du Solide Mine ´ral-UMR 7555, Universite ´ Henri Poincare ´ Nancy I, B.P. 239, 54506 Vandoeuvre-le `s-Nancy Cedex, France 4 Institut de Mine ´ralogie et de Physique des Milieux Condense ´s-CNRS, Universite ´ Pierre et Marie Curie-Paris 6, 4, place Jussieu, 75252, Paris, France (Received 2 February 2006; revised manuscript received 24 March 2006; published 6 February 2007) We measured the temperature dependent resistivity, %T, of the intercalated graphite superconductor CaC 6 as a function of pressure up to 16 GPa. We found a large linear increase of critical temperature, T c , from the ambient pressure value 11.5 K up to 15.1 K, the largest value for intercalated graphite, at 7.5 GPa. At 8 GPa, a jump of % and a sudden drop of T c down to 5K indicates the occurrence of a phase transition. Our data analysis suggests that a pressure-induced phonon softening related to an in-plane Ca phonon mode is responsible for the T c increase and that higher pressures *8 GPa lead to a structural transition into a new phase with a low T c & 3K. DOI: 10.1103/PhysRevLett.98.067002 PACS numbers: 74.62.Fj, 74.25.Fy, 74.70.b Graphite intercalated compounds (GICs) have attracted a great deal of interest, for graphene is a model system of two-dimensional electron gas whose electronic properties can be radically altered through intercalation. Despite the low critical temperatures, T c & 5K, hitherto reported [1], early findings of superconductivity in GICs stimulated intensive research efforts in the hope of raising T c through an effective carrier doping of the graphene layer. The interest in the topic has been renewed after the discovery of superconductivity at 39 K in MgB 6 [2], characterized by a similar honeycomb layer structure. A breakthrough came recently with the discovery that CaC 6 is superconducting with T c  11:5K [3,4], which raises the question of the origin of such unusually high T c for GICs and stimulates the search of compounds with even higher T c . For CaC 6 , magnetic penetration depth measurements [5] and ab initio calculations [6,7] point at a conventional BCS scenario with a medium electron-phonon coupling,   0:83, and with an s-wave superconducting gap, , with 2=T c  3:6. These calculations indicate that Ca radically alters the band structure and the phonon modes relevant to , con- trary to a simple picture of rigid band filling. In order to verify the possibility of further raising T c and to study the role of Ca phonons in the transport and super- conducting properties of CaC 6 , in this Letter we studied the temperature dependence of resistivity, %T, of high- quality bulk CaC 6 samples at ambient and high pressure up to 16 GPa. To our knowledge, this is the first study of transport properties on CaC 6 . Our results show a large pressure-induced increase of T c up to 15.1 K, the highest value hitherto reported for GICs, followed by a sudden drop to 5 K at  8 GPa. Our data analysis suggests that the T c increase arises from a pressure-induced enhancement of  that leads, at higher pressures, to a structural instability with the formation of a lower T c phase. We measured three bulk CaC 6 samples of 1 mm size prepared from platelets of c-axis oriented pyrolithic graph- ite, as described elsewhere [8]. The %T measurements were carried out in a four-probe bar configuration using a dc method. Because of the reactivity of CaC 6 , the contacts were made using silver paste in a glove box. The samples were subsequently protected by halogen-free cryogenic grease to enable handling in air. Owing to their shape and orientation, the in-plane % was measured. The ambient pressure measurements prior and after pressurization were carried out in a commercial quantum design physical prop- erty measurement system (PPMS). For the high-pressure study, we used a cubic anvil press enabling the four-probe measurement of %T under hydrostatic conditions up to 16 GPa and down to 2.5 K [9]. The sample was placed in a teflon capsule filled with fluorinert liquid used as pressure- transmitting medium. Each run of measurements was car- ried out at constant pressure on cooling and heating by adjusting the load. The ambient pressure results are shown in Fig. 1. Note the low values of room temperature and residual re- sistivities, % 300 K  46 cm and % 0  0:8 cm, re- spectively, the large residual resistivity ratio, RRR  % 300 K =% 0  58. The T dependence gradually approaches a linear behavior at high temperature, as evident in loga- rithmic scale. This dependence is radically different from that of graphite, although the % 300 K value is similar to that of graphite single crystals [10] or pyrolithic graphite [11]. First, in graphite, RRR is much smaller and 15 even in PRL 98, 067002 (2007) PHYSICAL REVIEW LETTERS week ending 9 FEBRUARY 2007 0031-9007= 07=98(6)=067002(4) 067002-1  2007 The American Physical Society